This invention relates to a fuel pump. In particular the invention relates to a pump for use in a fuel system for an engine of the compression ignition type. The fuel pump of the invention is particularly suitable for use in a common rail type fuel system. It will be appreciated, however, that the pump is also suitable for use in other applications.
In a common rail fuel system, a high pressure fuel pump is used to charge a common rail to a high pressure. A plurality of individually actuable fuel injectors are arranged to control the supply of fuel from the common rail to the combustion spaces of an engine. The fuel pump is typically a cam actuated plunger pump.
Difficulties have been faced in achieving filling of the pump chambers of the pump in the time available, particularly where the flow of fuel to the pump is metered, and as a result a feed pump, conveniently located in the fuel tank, or a transfer pump has often been provided. Such transfer or feed pumps increase the cost and complexity of the fuel system and result in the system being of increased dimensions. In order to increase the range of applications for which a pump may be used, it is thought to be important to minimise the axial length of the pump.
is an object of the invention to provide a fuel pump in which the provision of a separate transfer or feed pump can be avoided. It is a further object to provide a fuel pump of relatively short axial extent.
According to the present invention there is provided a fuel pump comprising a first pumping plunger reciprocable within a first bore and defining, with the first bore, a first pumping chamber, a second pumping plunger reciprocable within a second bore and defining, with the second bore, a second pumping chamber, a supply passage whereby fuel from the first pumping chamber is supplied to the second pumping chamber, and drive means for driving the plungers in both an extending direction and a retracting direction such that when the drive means drives the first plunger in its retracting direction, the second plunger is driven in its extending direction to expel fuel from the second pumping chamber, and when the drive means drives the first plunger in its extending direction, the second plunger is driven in its retracting direction and fuel is expelled from the first pumping chamber through the supply passage to the second pumping chamber.
By using a first reciprocable plunger to charge a second pumping chamber in this manner, the provision of a separate transfer or feed pump can be avoided.
The first and second plungers are conveniently axially aligned. The plungers are conveniently driven by a common cam arrangement and are conveniently interconnected. Such an arrangement permits the axial extent of the pump to be reduced.
Alternatively, the first and second plungers may be spaced apart in the axial direction of the pump. In such an arrangement, the plungers are conveniently driven by separate cam arrangements, a pivotable drive member being used to drive the plungers in the retracting direction.
In a preferred embodiment of the present invention, the drive means comprise first and second tappet members associated with the first and second pumping plungers respectively, the first and second tappet members being reciprocable within first and second tappet bores respectively, the first and second tappet members defining, together with the respective tappet bore, first and second further chambers, the volumes of the first and second further chambers varying, in use, as the tappet members reciprocate within their respective tappet bores, the first and second further chambers being provided with vent means for venting the first and second chambers so as to substantially prevent reciprocating movement of the first and second tappet members being impaired.
Conveniently, the pump may further comprise means for permitting air to be drawn into the first and second further chambers.
The pump preferably comprises a metering arrangement for controlling the quantity of fuel supplied to the first and second pumping chambers.
Conveniently, the pump comprises a filling passage through which fuel flows into the first pumping chamber, the metering arrangement being provided within the filling passage, and an accumulator for fuel being provided to assist filling of the first pumping chamber.
According to another aspect of the invention there is provided a fuel pump comprising a pumping plunger reciprocable within a bore and a drive arrangement for driving the pumping plunger in both an extending direction and in a retracting direction, wherein the drive arrangement comprises a rotatable cam and a pair of cam followers coupled to one another and to the pumping plunger.